Polybenzimidazoles (PBI), particularly aromatic PBIs, are characterized by a high degree of thermal stability and a great resistance to degradation by heat and hydrolytic and oxidizing media.
PBIs are not easy to make and once made, they are difficult to process. PBIs are made by a melt condensation reaction where monomers are polymerized, at high temperatures, to form a friable foam, the foam is crushed, and optionally the crushed PBI is further polymerized in a solid state. Since it is highly heat stable, the PBI resin must be dissolved, in the organic solvents mentioned below, for subsequent use, such as coatings and shaped articles (e.g., fibers, films, membranes, sheets, rods, billets, slabs).
PBIs are most commonly dissolved, by heating, in highly polar, aprotic organic solvents. Those solvents include: N,N-dimethylacetamide (DMAc); N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO), and N-methyl-2-pyrrolidone (NMP). While these solvents are effective, they are most often considered environmentaly unfriendly. See generally, Wang, B., et al., Dissolution and regeneration of polybenzimidazoles using ionic liquids, European Polymer Journal, EPJ 45 (2009) pages 2962-2965.
Recently, it has been reported that PBIs may be dissolved in certain ionic liquids. Wang, et al., Ibid. PBI is reported to have dissolved in 1-butyl-3-methylimidazolium chloride ([BMIM]Cl), 1-butyl-3-methylimidazolium hydroxide ([BMIM]OH), and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4).
Accordingly, there is a need for new solvents for PBI. Such solvents could replace the typically used organic solvents for the preparation of PBI solutions.